Mitochondria are double membrane-bound organelles that govern ATP production and many other important cellular processes. Mitochondrial membranes undergo frequent fusion to maintain a ribbon-like morphology and actively divide for clearance of damaged portions or redistribution during cell division. These membrane dynamics are mediated primarily by dynamin-like proteins (DLPs). In mammals, mitofusin (MFN) fuses outer mitochondrial membranes. Two MFN isoforms have been identified in mammalian cells, MFN1/2. Deletion of either MFN in mice causes embryonic lethality and mitochondrial fragmentation. Human mutations in MFN2 are associated with neurodegenerative disease Charcot-Marie-Tooth neuropathy type 2A (CMT2A). Given the difficulties purifying and reconstituting full-length MFN proteins, how MFN-mediated mitochondrial fusion occurs is poorly understood.

On November 1st, 2017, a study led by Prof. Junjie Hu from Institute of Biophysics (IBP) of the Chinese Academy of Sciences (CAS) was published in PNAS as a PLUS article. It is entitled “Sequences flanking the transmembrane segments facilitate mitochondrial localization and membrane fusion by mitofusin”. Their studies revealed that MFN1 is a bona fide fusogen without requirement for mitochondrial-specific cofactors. They also identified the necessary elements for MFN1’s mitochondrial targeting. In addition, they showed that an amphipathic helix in the CT of MFN1 is critical for its fusion ability. These findings provide important insight into MFN-mediated membrane fusion.

According to sequence comparison and secondary structure prediction, it is found that MFN1 share very similar domain structures with Atlastin (ATL), an ER membrane protein mainly mediating ER membrane fusion. They both comprise an N-terminal cytosolic GTPase followed by a helical bundle (HB) domain, a transmembrane (TM) domain, and a cytosolic C-terminal tail (CT). Given the function mechanism of ATL1 has been studied much clearly, the researchers engineered various MFN-ATL chimeras to gain mechanistic insight into MFN-mediated fusion. When MFN1 is localized to the ER by TM swapping with ATL1, it functions in the maintenance of ER morphology and fusion. An amphipathic helix in the CT of MFN1 is critical for its fusion ability as well as its properly mitochondrial targeting. In addition, hydrophobic residues N-terminal to the TM segments of MFN1 also play a role in membrane targeting.

These mechanistic probing of MFN1 demonstrates the dependence of each domain and the specific role of the CT in mediating membrane fusion. It also provides insight into the molecular determinants of the localization of MFN1. The mechanism proposed for MFN1 could apply to MFN2, Fuzzy Onion, Fzo1p, and their homologs in the fusion of mitochondrial outer membranes in different tissues or species.

This work is supported by National Key Research and Development Program, National Natural Science Foundation of China, and an International Early Career Scientist grant from the Howard Hughes Medical Institute.